mid-February 2019 (NET February 18?) on reused Falcon 9 from Vandenberg. Mass is around 1400kg? for each of the three satellites? plus a custom deployer. Orbit is 600km SSO.

[ Edit Oct. 23, 2017: Most of the references to this mission still have it launching in the second half of 2018. Except for that one comment from DLR that they were calibrating the transponders leading up to launch in 2019. The mass is generally listed as about 1400kg (total?), except for that one saying it's 1600kg. ]

Does SpaceX have a suitable standard multiple payload dispenser in development or will a one off hardware solution for deploying three satellites need to be devised for this mission?

SpaceX have a deal with Spaceflight Inc. to supply a Seconday Payload System for the Falcon 9, but this I believe is for small secondary sats only. Whether they will outsource the payload dispenser for this mission as well is anyone's guess.